MED and community fluctuation

Daniel Roesen dr at
Fri Oct 8 18:49:22 UTC 2004

On Fri, Oct 08, 2004 at 11:40:54AM -0700, Zhen Wu wrote:
> We are thinking of the motivation of doing this?

Traffic enginneering.

> Why the ISPs configured their network so that the MED values
> oscillate?

Is there actually persistant oscillation, or just "frequent change"
with some peers at some periods of time?

> Did ISPs configure MED values dynamically calculated from IGP metrics?

This may very well the reason. At least Cisco and Juniper offer the
option to derive MED from IGP metric.

> The most strange thing to us is why the community attribute associated 
> with prefixes also oscillate frequently. [...]
> Misconfigurations?

Possibly. Routes received via different ingress points being tagged
differently (e.g. country/city based scheme) and due to backbone
topology changes, a different "version" of the route becoming "best"
and thus announced to the looking glass. Or routes received via
multiple ingress points and some missing the ingress tagging due
to misconfigured route-map/policies. Same outcome.

Best regards,

CLUE-RIPE -- Jabber: dr at -- dr at IRCnet -- PGP: 0xA85C8AA0

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